Late Cambrian-Early Ordovician Research Articles

Protracted oxygenation across the Cambrian-Ordovician transition: A key initiator of the Great Ordovician Biodiversification Event?

Fluctuations in marine oxygen concentrations have been invoked as a primary driver for changes in biodiversity throughout Earth history. Expansions in reducing marine conditions are commonly invoked as key causal mechanisms for mass extinctions, while increases in marine oxygenation are becoming an increasingly common causal mechanism invoked for biodiversification events. Here we utilize a multiproxy approach to constrain local and global marine paleoredox conditions throughout the late Cambrian-Early Ordovician from two drill core successions in Baltoscandia. Local paleoredox proxies such as manganese concentrations and iron speciation reveal that both sites in the Baltic paleobasin had persistently anoxic and predominantly euxinic (anoxic and sulfidic) bottom water conditions throughout the study interval. Corresponding trace metal datasets indicate nuanced contraction and expansion of global anoxic and euxinic conditions along continental margins during the late Cambrian-Early Ordovician. Lastly, thallium isotope data from these locally reducing sections suggest a global expansion of oxygenated shelf and deeper marine environments from the late Cambrian into the Early Ordovician. This evidence for increasingly oxic marine environments coincides with increases in burrowing depth and tiering in marine animals, as well as diversification of body fossils throughout this ~8-million-year interval. The collective geochemical datasets provide some of the first direct paleoredox evidence for an increase in marine oxygen concentrations as a key mechanism for the Ordovician radiation of marine life.

Первые U/Pb-данные о возрасте детритового циркона из песчаников золотоносной верхнекембрийско-нижнеордовикской алькесвожской толщи (Приполярный Урал)

The paper presents the novel isotope data on the age of sandstones of the gold-bearing Alkesvozhskaya sequence at the base of the Paleozoic section in the Subpolar Urals. The analysis of U/Pb dating of detrital zircons confirmed that the sandstones had been formed no earlier than in the Late Cambrian-Early Ordovician. The predominant part of dating is in a narrow Late Riphean-Late Cambrian age interval, and zircons are represented by unrounded or weakly rounded crystals. The composition of the sandstones of the Alkesvozhskaya sequence are dominated by the destruction products of the underlying rocks of igneous complexes at various stages of the formation of timanides-protouralides.

Occurrence and origin of glauconite in the Cambro-Ordovician Bliss Formation of southern New Mexico and West Texas (U.S.A.)

ABSTRACTIn southern New Mexico and West Texas, USA, the Late Cambrian–Early Ordovician Bliss Formation is a relatively thin and dominantly siliciclastic succession that was deposited on the Proterozoic basement during a major global transgression. The Bliss Formation can be divided into two members: 1) a lower, coarser-grained member composed mostly of sandstone (quartz arenite and subarkose) that lacks glauconite and calcite cement, and 2) an upper, finer-grained member that includes glauconitic sandstone, arkosic sandstone, and mixed siliciclastic–carbonate siltstone to fine-grained sandstone, intercalated with thin carbonate beds of grainstone, packstone, and rudstone. Iron oolite and oolitic sandstone are locally exposed at the base of the upper member. Sandstone of the lower member represents upper-shoreface to foreshore deposits, whereas sedimentary structures in the upper member indicate deposition in a middle- to lower-shoreface setting, and locally in a tidal-flat environment. Intercalated carbonate beds are storm layers (tempestites). Glauconite grains are abundant in the upper member as mostly rounded to well-rounded, spheroidal to ovoidal pellets. Two types of glauconite grains are present: homogeneous, dark green grains with high K2O contents (> 8 wt.%; stage 4) and mottled pellets composed of a mixture of glauconite and apatite. Glauconite of the Bliss Formation is not autochthonous as proposed by earlier workers, but of allochthonous (parautochthonous) origin. Thus, the glauconite grains were reworked from deeper shelf environments in northern Mexico to the south and were transported and deposited under regressive–transgressive conditions. Mottled glauconite grains formed by the reworking of phosphatized and glauconitized micritic sediments during regression and were transported and deposited by storm-induced currents, particularly in storm layers during transgressive events. Homogeneous, mature glauconite grains probably were derived from the reworking of glauconitized fecal pellets or completely glauconitized micritic sediments during regressive–transgressive cycles. The world-wide occurrence of glauconite in Cambrian–Early Ordovician sediments indicates that glauconite formation during that period can be considered as a “global event.”

Multiple phases of deformation in the southern Helanshan tectonic Belt, northern China

Abstract The Southern Helanshan Tectonic Belt (SHTB) is surrounded by the Alxa Massif, the Ordos Basin and the Tibetan Plateau, and its Phanerozoic structures records crucial Phanerozoic deformation characteristics for studying the multi-phased intracontinental deformation processes among these neighboring units. Combined with regional geology, our structural analysis reveals that the SHTB has experienced five phases of shortening deformation during the Phanerozoic. The first phase (D1) is represented by E-W-trending overturned folds in the Cambrian and Ordovician strata. These folds resulted from two episodes of N-S shortening during the Late Cambrian-Early Ordovician and Late Ordovician-Early Devonian respectively, which were attributed to the closure of the Qilian ocean. The second phase (D2) is characterized by E-W-trending open folds involving the Upper Devonian strata, which was the products of N-S shortening caused by the assembly of the Qilian ocean and the North China Craton (NCC) at the end of Devonian. The third phase (D3) is indicated by some WNW-trending Jura-type folds in the Carboniferous strata, formed by NNE-SSW shortening during the Late Triassic-Early Jurassic related to the collision between the NCC and the South China Block. The fourth phase (D4) is inferred by NNE-trending shortening structures in the Jurassic and its underlying strata, generated by WNW-ESE shortening in the Late Jurassic echoing the Paleo-Pacific subduction. The fifth stage (D5) is characterized by the NE-SW shortening in the Late Miocene, leading to the formation of many NW-SE striking thrust faults in the Miocene and its underlying strata, because the outward expansion of the Tibetan Plateau.

Tectonic controls on late Cambrian-Early Ordovician deposition in Cordillera oriental (Northwest Argentina)

The western Gondwana margin underwent a complex geodynamic history during the early Paleozoic, and major uncertainties remain as to the role of tectonism in sedimentary dynamics. This study focuses on the lower part Santa Rosita Formation and the coeval Guayoc Chico Group (Cordillera Oriental; Northwest Argentina), ranging from the late Cambrian (Furongian; Age 10) to Early Ordovician (early Tremadocian; Tr1). This stratigraphic interval has been previously interpreted as deposited in an extensional basin to a retro-arc basin without major regional tectonic-induced deformation during its deposition, only recording long-term relative sea-level fluctuations. Four areas (Sierra de Cajas, Angosto del Moreno, Quebrada de Trancas, and Quebrada de Moya) were chosen because they host the most complete and temporally well-constrained stratigraphic sections of the Cordillera Oriental. Throughout the stratigraphic sections, four main facies zones are described and attributed to deposition in estuarine, foreshore-shoreface, delta-front, and offshore environments. Trilobite biozones are used as the biostratigraphic framework. By integrating sedimentary facies analysis, biostratigraphy, and sequence stratigraphy from the four selected sections, a new scenario showcasing the evolution of the basin is proposed. This scenario interprets a tectonically induced deformation during the deposition of the Santa Rosita Formation and the coeval Guayoc Chico Group. The newly acquired sedimentological data show that physiographical changes took place during the Cambrian-Ordovician transition and are expressed in various localities. This major change is recorded in the stratigraphic architecture, where extensive wave-ravinement surfaces and sedimentary hiatus are the result of local, syn-depositional basement uplifts. The initiation of the Puna-Famatinian volcanic arc during the Early Ordovician on the western margin was likely responsible for deformation in the retro-arc basin and the proposed scenario is consistent with the stratigraphic evolution in other areas of the Cordillera Oriental (e.g., Sierra de Mojotoro) and the Sierra de Famatina. Therefore, this study helps to constrain the evolution of the western Gondwana margin during the early Paleozoic, showing changes in the stratigraphic architecture and basin evolution from an extensional to a retro-arc style.

Early Paleozoic magmatism and metallogeny related to Proto-Tethys subduction: Insights from volcanic rocks in the northeastern Altyn Mountains, NW China

Remnants of the Proto-Tethys are mainly preserved in the region between south of the North China-Tarim Block and north of Qiangtang-Sibumasu/Baoshan Blocks. Magmatic-metallogenic events related to the Proto-Tethyan subductions were rarely reported, and the subduction history and polarity of the Proto-Tethyan are still under debate. Here, we presented new data of zircon UPb ages, whole-rock Sr–Nd–Pb isotopes, major and trace elements and zircon Hf isotopes for the volcanic rocks in the northeastern Altyn Mountains. Information of over 14 volcanic-hosted deposits/prospects in the region has been compiled. These volcanic ore hosts consist mainly of basaltic andesite, andesite, dacite and rhyolite rocks. The andesite and rhyolite rocks are newly zircon UPb dated to be Late Cambrian-Early Ordovician (andesite: 490.5 ± 5.2 Ma; rhyolite: 492.6 ± 2.9 Ma and 491.6 ± 5.6 Ma), representing the timing of volcanism and VMS (Volcanogenic Massive Sulfide) mineralization. All the volcanic rocks belong to the high-K calc-alkaline and shoshonite series: the andesite rocks from the Kaladawan area in north of the region display arc geochemical affinities and contain (87Sr/86Sr)i (0.7082–0.7083) and εNd(t) (−9.7 to −7.6), indicating that they were likely formed by partial melting of the mantle wedge with subducted sediment inputs. The rhyolite rocks from the Kaladaban area in south of the region are characterized by high SiO2 (64.46–78.55 wt%), low alkali (Na2O + K2O, 3.46–7.17 wt%), and contain (87Sr/86Sr)i (0.7063–0.7095), εNd(t) (−6.6 to −1.5), and zircon εHf(t) (−5.5 to 5.4), indicating that they were likely derived from partial melting of the lower crust with depleted mantle inputs. Rock assemblage and geochemistry suggest that volcanic rocks in the northeastern Altyn Mountains may have formed in a continental arc setting. Their spatial distributions with respect to the ophiolites in the region suggest that the subduction was likely south-dipping. This subduction-related arc magmatism may have formed the many important VMS and porphyry–skarn deposits in the region.

The age and petrologic and geochemical conditions of formation of the Kogtakh gabbro-monzonite complex in the Kuznetsk Alatau

We present new data on the U-Pb age of zircons, geochemistry of trace elements, and isotope (Nd, Sr, O) composition of rocks of the Kogtakh gabbro-monzonite complex on the eastern slope of the Kuznetsk Alatau Ridge. The established age of zircon in the rocks of the main intrusive phases (500-480 Ma) is taken as the time of formation of the Kogtakh complex in the Late Cambrian-Early Ordovician, during the accretion-collision stage of evolution of the Central Asian Orogenic Belt. The distribution and ratios of LILE and HFSE in the rocks suggest that the intrusions proceeded under interaction of the PREMA + EM/OIB mantle plume and the suprasubductional lithospheric IAB mantle. The mantle-crust interaction led to the heterogeneous isotopic composition of neodymium in the magma source (ɛNd(T) ~ 3.5-5.4). The mixing of different mantle and continental-crust materials resulted in an increase in isotope parameters: (87Sr/86Sr)T ~ 0.7039-0.7052, ɛSr(T) ~ 0-19, and δ18O ~ 6.5-8.8‰ (SMOW). The REE ratio in the least differentiated gabbroids indicates different depths of probable magma chambers and the formation of their primary (Tb/YbPM > 1.8) mafic melts at different degrees of melting of the model garnet peridotite.

Mineralogy and spectroscopy of Owen Group sandstones, Australia: implications for the provenance, diagenesis, and origin of coloration

Mineralogical studies were conducted on sandstones of Late Cambrian-Early Ordovician Owen Group, southern West Coast Range, Tasmania, Australia. Several samples were analysed using conventional thin section and point counting techniques, while others were further analysed under a Scanning Electron Microscope with back scattering and Energy Dispersive X-ray Spectroscopy capabilities. The result of petrographic analyses includes provenance, possible sources and controls over coloration in sandstone samples. The sandstones are mainly comprised of quartz, mica, lithic grains (volcanic, metamorphic and sedimentary), clays, zircon, tourmaline, leucoxene, rutile, and chert grains. These were derived from a Proterozoic Tyennan metamorphic and a Middle Cambrian Mount Read Volcanic provenance. Following deposition, sediments underwent severe diagenesis including quartz overgrowth, iron coatings, haematite, chlorite and sericite alterations, linings and cements. Multi-phase haematite-related diagenesis is present and late stage hydrothermal fluid alterations assemblages have severely altered and colored the sediments.

Geochemistry and metamorphism of the Mouriscas Complex, Ossa-Morena/Central Iberian zone boundary, Iberian Massif, Central Portugal: Implications for the Cadomian and Variscan orogenies

The Mouriscas Complex is a deformed and metamorphosed predominantly mafic igneous complex of Ediacaran and Ordovician age and crops out at the Ossa-Morena/Central Iberian zone boundary in the Iberian Massif, Central Portugal. It comprises amphibolite with Neoproterozoic protoliths (ca. 544Ma), protomylonitic felsic dykes derived from younger trondhjemitic protoliths (ca. 483Ma) and garnet amphibolite derived of even younger dioritic protoliths (ca. 477Ma). The protoliths of the Neoproterozoic amphibolites are calc-alkaline magmas of basic to intermediate compositions with intraplate and active continental margin affinities and are considered to represent the final phase of the Cadomian arc magmatism. They are interpreted to have originated as coarse-grained intrusions, likely gabbro or diorite and generated from the partial melting of meta-igneous lower crust and mantle. Their emplacement occurred near the Cadomian metamorphic event dated at ca. 540Ma (P=7–8kbar and T=640–660°C) which is interpreted to represent a continental collision. During the Late Cambrian-Early Ordovician an extensional episode occurred in the central-southern Iberian Massif and was also observed in other areas of the Variscan Orogen. It led to mantle upwelling and to the development of an aborted intracratonic rift located at the Ossa-Morena/Central Iberian zone boundary and to the opening of the Rheic Ocean to the south of the area studied in present coordinates (i.e., between the Ossa-Morena and South Portuguese Zones). This event has been dated at ca. 477Ma and was responsible for the melting of deep ancient mafic crust and mantle with formation of bimodal magmatism in an intra-plate setting, as indicated by the protoliths of the protomylonitic felsic dykes with trondhjemitic composition and of the garnet amphibolite. Subsequent Variscan metamorphism took place under amphibolite facies conditions (P=4–5.5kbar; T=600–625°C) at lower P-T conditions than the Cadomian metamorphic event. It was followed by greenschist retrogression as suggested by the appearance of actinolite rims and formation of chlorite and epidote.

Provenance sources of Upper Proterozoic and Lower Paleozoic clastic deposits of the Lesser Khingan terrane, Central Asian fold belt: U–Pb (LA–ICP–MS) geochronological results

The ages of detrital zircons from sandstones and silty sandstones from conditionally Upper Proterozoic–Cambrian rocks have been obtained for the first time. These deposits are united in the Khingan Group and are one of the most ancient formations in the Lesser Khingan terrane in the eastern part of the Central Asian fold belt. These data along with relationships with granitoids imply that the ages of the Muranda and underlying Igincha Formations range within the Tonian–Ediacaran time span, while the age of the Kimkan unit is Late Cambrian–Early Ordovician. Given that the Muranda and Igincha Formations, on the one hand, and the Kimkan unit, on the other hand, are substantially different in age and structurally separated by granitoids intruded at the Ediacaran–Cambrian boundary, we should accept that they cannot be regarded as a single sedimentary sequence.

Evolution of the Proto-Tethys in the Baoshan block along the East Gondwana margin: constraints from early Palaeozoic magmatism

ABSTRACTZircon U–Pb ages and geochemical and isotopic data for Late Ordovician granites in the Baoshan Block reveal the early Palaeozoic tectonic evolution of the margin of East Gondwana. The granites are high-K, calc-alkaline, metaluminous to strongly peraluminous rocks with A/CNK values of 0.93–1.18, are enriched in SiO2, K2O, and Rb, and depleted in Nb, P, Ti, Eu, and heavy rare earth elements, which indicates the crystallization fractionation of the granitic magma. Zircon U–Pb dating indicates that they formed at ca. 445 Ma. High initial 87Sr/86Sr ratios of 0.719761–0.726754, negative ɛNd(t) values of –6.6 to –8.3, and two-stage model ages of 1.52–1.64 Ga suggest a crustal origin, with the magmas derived from the partial melting of ancient metagreywacke at high temperature. A synthesis of data for the early Palaeozoic igneous rocks in the Baoshan Block and adjacent Tengchong Block indicates two stages of flare-up of granitic and mafic magmatism caused by different tectonic settings along the East Gondwana margin. Late Cambrian to Early Ordovician granitic rocks (ca. 490 Ma) were produced when underplated mafic magmas induced crustal melting along the margin of East Gondwana related to the break-off of subducted Proto-Tethyan oceanic slab. In addition, the cession of the mafic magmatism between late Cambrian-Early Ordovician and Late Ordovician could have been caused by the collision of the Baoshan Block and outward micro-continent along the margin of East Gondwana and crust and lithosphere thickening. The Late Ordovician granites in the Baoshan Block were produced in an extensional setting resulting from the delamination of an already thickened crust and lithospheric mantle followed by the injection of synchronous mafic magma.

Tectonic conditions of sedimentation and source areas of Upper Proterozoic and Lower Paleozoic terrigenous deposits of the Lesser Khingan Terrane of the Central Asian Fold Belt

This work presents the results of geological, geochemical, and Sm-Nd isotopic and geochemical studies of Late Riphean–Cambrian terrigenous rocks of the Khingan Group of the Lesser Khingan Terrane of the Central Asian Fold Belt, as well as the results of U-Pb geochronological (LA-ICP-MS) studies of detrital zircons from these deposits. These deposits are the most ancient in the structure of the terrain. It was found that the deposits of Iginchi and underlying Murandavi formations are attributed to the Late Riphean–Vendian age interval, and the Kimkan sequence, to the Late Cambrian–Early Ordovician. The periods of formation of the Murandavi and Iginchi formations, on one hand, and the Kimkan sequence, on the other hand, are separated by the stage of granitoid magmatism at the turn of the Vendian–Cambrian. Because of this, they cannot be attributed to a unified sedimentary sequence. It is the most probable that the sedimentation of the Iginchi and Murandavi formations and the Kimkan sequence occurred under subduction conditions against the backdrop of magmatic activity.

Improved Geochronologic Accuracy and Precision for the ICS Chronostratigraphic Charts: Examples from the late Cambrian–Early Ordovician

The utility of ICS International Chronostratigraphic Charts (ICS Charts) relies on the accuracy and precision of reported geochronologic dates and their relationship to the bases of global chronostratigraphic units. However, an examination of some early Paleozoic dates (i.e., Cambrian-Ordovician boundary and lowest Ordovician) on the ICS Charts shows that they differ from originally reported and late recalculated dates. Thus, they record a numerical precision that differs from these revised dates. The dates assigned to the bases of formal early Paleozoic chronostratigraphic units may be interpreted by the lay reader as precise determinations, although they are almost all estimates based on dated horizons that stratigraphically bracket a chronostratigraphic unit's base. The bases of informal Cambrian chronostratigraphic units (e.g., Stage 10, ∼489.5 Ma) are also assigned relatively precise ages although consensus does not exist on the horizon that should define an informal unit's base, and several alternative GSSP horizons with differing ages may exist. The uncertainties of the ICS Chart's dates on stage bases within the traditional Upper Cambrian are so great that the tilde (e.g., ∼500.5) indicating about might be replaced by question marks or an abbreviation for estimated. A preferred alternative is to replace the age estimates on the bases of informal and almost all formal chronostratigraphic divisions in the Phanerozoic with precise dates that bracket the bases of the divisions. The ICS Chart should include (perhaps on-line) brief discussions on and references to the history and procedures of age determination, accuracy, and precision for each dated horizon. An even more accurate, precise, and informative ICS Chart would result from review by a new ICS subcommission on geochronology. The subcommission could evaluate proposed dates for the ICS Chart for accuracy, precision, and suitability for determining/bracketing the ages of chronostratigraphic unit bases and provide updated reviews of the procedures used to determine all ICS Chart dates.

Main stages in the evolution and geodynamic setting of the South Hangay metamorphic belt, Central Asia

This paper reports new geological, geochronological (U-Pb method) and Sm-Nd isotope-geochemical data on the metamorphic and magmatic complexes of the South Hangay metamorphic belt located in the southern part of the Hangay group of the Precambrian blocks, along the junction zone between the Baidarik block and the Vendian-Late Riphean ophiolite complex of the Bayanhongor zone. A lower age limit of the early metamorphic episode of the disthene-sillimanite facies series (M2) is provided by the age of 603 ± 3 Ma obtained for gabbroids of the island arc association distinguished in the Tatsaingol complex. An upper age limit is determined by granites with ages of 562 ± 2, 564 ± 5, and 571 ± 9 Ma. Later superimposed metamorphism of the andalusite-sillimanite facies series (M2) was established in the southeastern part of the belt. The lower boundary of this metamorphism is constrained by the emplacement of quartz diorites at 552 ± 2 Ma, while the upper boundary is marked by postkinematic granites of the Tatsaingol pluton with an age of 545 ± 5 Ma. The protoliths of high grade metamorphic rocks contained Early and Late Precambrian rocks. The former could belong to the basement of the Baidarik block and/or its shelf, while the latter, to the rocks of the oceanic and island-arc complexes of the Bayanhongor paleobasin. Virtually, the continental crust, fragments of which are represented in the Tatsaingol complex, was formed within an interval around 610-560 Ma. The final stages in the formation of the South Hangay metamorphic belt coincided with the initiation of the Vendian-Early Paleozoic paleoocenic basins, volcanic islands, and island arcs in the Early Caledonian framing of the Precambrian Hangay block structure in the Late Vendian. By the time of amalgamation of structures of the Early Caledonian superterrane of Central Asia in the Late Cambrian-Early Ordovician (500-470 Ma), an “agglomerate” of the Precambrian Hangay blocks has been already formed.

Biofacies and palaeoenvironments of conodonts in Cambro‐Ordovician sequences of the Quebrada de Humahuaca, Cordillera Oriental of Jujuy, Argentina

The taxa frequencies and cluster analysis of 10 837 conodonts recovered from the Santa Rosita Formation on the eastern flank of the Quebrada de Humahuaca, Cordillera Oriental of Argentina, were carried out for palaeoenvironmental interpretations. The first type of analysis allowed us to identify three conodont biofacies: Variabiloconus‐Teridontus, Utahconus‐Acanthodus and Tilcarodus‐Drepanoistodus, and the cluster analysis helps define respective sub‐biofacies. The first biofacies is not constrained to a particular environment, the second biofacies, which is characterized by typical Laurentian genera, is related to sandstones from shallow‐water environments, while the third one is better represented in deeper water siliciclastic lithofacies. A nektobenthic mode of life is suggested for Utahconus and Acanthodus, but a pelagic behaviour is apparent for the rest of the taxa, well adapted to off‐shore biotopes in particular cases (proto‐ and paraconodonts). The faunal composition reveals a mixture of Baltic and Laurentian taxa, as well as endemic forms that define the Southwestern Gondwana Province from the Cold Domain in the Shallow‐Sea Realm. The presence of typical species from low latitudes (e.g. Australia) confirms the installation of an oceanic perigondwanian corridor, which was open to faunal migration during the late Cambrian–early Ordovician. Copyright © 2012 John Wiley & Sons, Ltd.

Late Cambrian-Early Ordovician condensed oceanic siliceous deposits in the Iradyr zone, Northern Kazakhstan

Late Cambrian-Early Ordovician condensed oceanic siliceous deposits in the Iradyr zone, Northern Kazakhstan

Collisional granitoids of the Dzhida zone of the Central Asian Fold belt, Southwestern Transbaikalia: Age and conditions of the formation

This paper considers the geological structure, composition, and age of the Darkhintui, Barun-Gol, and Khuldat granitoid plutons of the Dzhida zone of Caledonides of the Central Asian Fold belts. These plutons were formed in the Late Cambrian-Early Ordovician in the range between 490 ± 2 and 477 ± 6 Ma, after tectonic juxtaposition of the oceanic and island-arc complexes of the Dzhida Zone and volcanogenic-carbonate-terrigenous rocks of the Khamardaban zone, i.e., at the collisional stage of the region evolution. Geological, geochronological, geochemical, and Nd isotope data indicate that the collisional granitoids of the Dzhida zone were derived by melting of continental crust thickened through accretion. The sources for parental melts of the granitoids were presumably Vendian-Early Cambrian juvenile igneous rocks of ophiolite and island-arc complexes, as well as the crustal material of the Lower Paleozoic flyschoid sediments of the back-arc basin of the Dzhida zone and metaterrigenous rocks of the Khamardaban zone.

GEOCHEMISTRY AND U-PB ZIRCON AGE OF THE INTERNAL OSSA-MORENA ZONE OPHIOLITE SEQUENCES: A REMNANT OF RHEIC OCEAN IN SW IBERIA

The Early Paleozoic geodynamic evolution in SW Iberia is believed to have been dominated by the opening of the Rheic Ocean. The Rheic Ocean is generally accepted to have resulted from the drift of peri-Gondwanan terranes such as Avalonia from the northern margin of Gondwana during Late CambrianEarly Ordovician times. The closure of the Rheic Ocean was the final result of a continent-continent collision between Gondwana and Laurussia that produced the Variscan orogen. The Ossa-Morena Zone is a peri-Gondwana terrane, which preserves spread fragments of ophiolites - the Internal Ossa-Morena Zones Ophiolite Sequences (IOMZOS). The final patchwork of the IOMZOS shows a complete oceanic lithospheric sequence with geochemical characteristics similar to the ocean-floor basalts, without any orogenic fingerprint and/or crustal contamination. The IOMZOS were obducted and imbricated with high pressure lithologies. Based on structural, petrological and whole-rock geochemical data, the authors argue that the IOMZOS represent fragments of the oceanic lithosphere from the Rheic Ocean. Zircon SHRIMP U-Pb geochronological data on metagabbros point to an age of ca. 480 Ma for IOMZOS, providing evidence of a well-developed ocean in SW Iberia during this period, reinforcing the interpretation of the Rheic Ocean as a wide ocean among the peri-Gondwanan terranes during Early Ordovician times.

The Ordovician Biodiversification: revolution in the oceanic trophic chain

The Early Palaeozoic phytoplankton (acritarch) radiation paralleled a long-term increase in sea level between the Early Cambrian and the Late Ordovician. In the Late Cambrian, after the SPICE delta C-13(carb) excursion, acritarchs underwent a major change in morphological disparity and their taxonomical diversity increased to reach highest values during the Middle Ordovician (Darriwilian). This highest phytoplankton diversity of the Palaeozoic was possibly the result of palaeogeography (greatest continental dispersal) and major orogenic and volcanic activity, which provided maximum ecospace and large amounts of nutrients. With its warm climate and high atmospheric CO2 levels, the Ordovician was similar to the Cretaceous: a period when phytoplankton diversity was at its maximum during the Mesozoic. With increased phytoplankton availability in the Late Cambrian and Ordovician a radiation of zooplanktonic organisms took place at the same time as a major diversification of suspension feeders. In addition, planktotrophy originated in invertebrate larvae during the Late Cambrian-Early Ordovician. These important changes in the trophic chain can be considered as a major palaeoecological revolution (part of the rise of the Palaeozoic Evolutionary Fauna of Sepkoski). There is now sufficient evidence that this trophic chain revolution was related to the diversification of the phytoplankton, of which the organic-walled fraction is partly preserved.

LATE CAMBRIAN AND EARLY ORDOVICIAN STEM GROUP CHITONS (MOLLUSCA: POLYPLACOPHORA) FROM UTAH AND MISSOURI

Abundant silicified shell plates (valves) of some of the oldest-known chitons were recovered from the Late Cambrian Notch Peak Formation of Utah. The chiton fauna is dominated numerically by Matthevia wahwahensis new species, but also includes another mattheviid, Eukteanochiton milleri new genus and species, and a preacanthochitonid, Orthriochiton utahensis new genus and species. Robustum from the Early Ordovician Gasconade Formation of Missouri is herein reinterpreted as a septemchitonid chiton, and Hemithecella, from the Late Cambrian-Early Ordovician of the eastern United States, is once again considered a mattheviid chiton. Mattheviid valves are unique among chitons in that they are massive, elongate, and contain one or two tunnels; these characteristics have led some to exclude this family from the Polyplacophora. However, mattheviids and other chitons share many valve characters, including granules, an apical shelf, a thin anterior margin, bilateral symmetry, three valve types, and a shell layer perforated with canals. Furthermore, recently described chiton valves from the Silurian of Gotland, the Ordovician of Wisconsin, and the Cambrian-Ordovician of Missouri are gradational between the Notch Peak mattheviid valves and those of younger polyplacophorans. Also, Matthevia wahwahensis n. sp. has valves with a flat apical area, allowing for valve overlap, and so provides a link between the unusual chiton Matthevia variabilis, which had nonoverlapping, spiky valves, and Chelodes, a less disputed chiton. Known fossil assemblages reveal that, along the coastline of Laurentia during the Cambrian-Ordovician, chitons were diverse, consisting of at least four families and with much variation in valve shape.